This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-006577, filed Jan. 14, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to an electromagnetic clutch used in, for example, an automotive air conditioner.
An electromagnetic clutch that can alternatively select transmittal or non-transmittal of power by means of electromagnetic force is constructed in the manner shown in FIG. 7, for example. The electromagnetic clutch 101 comprises a clutch rotor 102 rotatable by means of an external drive source (not shown), ring armature 103 opposed to one end face of the rotor 102, ring exciting coil 104, ring field 105 containing the coil 104, return spring 113, etc. FIG. 7 shows a state in which current is supplied to the ring exciting coil 104 to generate a magnetic flux M that is diagrammatically indicated by a broken line.
As shown in FIG. 9, the clutch rotor 102 includes can outer tube portion 102a, an inner tube portion 102b, and a flange portion 102c that connects the tube portions 102a and 102b. The flange portion 102c is provided on one end portion of the clutch rotor 102 that faces the armature 103. The front face of the flange portion 102c serves as a friction surface that touches the armature 103.
As shown in FIG. 8, the flange portion 102c of the clutch rotor 102 is formed having arcuate outside slits 106a, 106b, 106c, 106d, 106e and 106f and arcuate inside slits 107a, 107b, 107c, 107d, 107e and 107f. Outside bridge portions 108a to 108f are arranged individually between the adjacent outside slits 106a to 106f. The outer peripheral portion of the flange portion 102c is supported on one end portion of the outer tube portion 102a by means of the bridge portions 108a to 108f. Further, inside bridge portions 109a to 109f are arranged individually between the adjacent inside slits 107a to 107f. The inner peripheral portion of the flange portion 102c is supported on one end portion of the inner tube portion 102b by means of the bridge portions 109a to 109f. 
When current is supplied from an external power source (not shown) to the exciting coil 104, the coil 104 generates the magnetic flux M, as shown in FIG. 7. The armature 103 is attracted to an end face of the clutch rotor 102 by means of an electromagnetic force based on the magnetic flux M. Thus, the rotor 102 and the armature 103 are connected magnetically to each other. When current supply to the coil 104 is interrupted, the magnetic flux M dies out, so that the armature 103 is separated from the end face of the rotor 102 by means of the elastic force of the return spring 113. The coil 104 is held in the ring field 105. The field 105 is held in a ring groove 110 that is defined by the respective inner surfaces of the outer tube portion 102a, inner tube portion 102b, and flange portion 102c. 
As shown in FIG. 7, the magnetic flux M gets out of the ring field 105 and gets into the outer tube portion 102a through a gap 111a between the field 105 and the outer tube portion 102a. The magnetic flux M in the outer tube portion 102a gets into the armature 103 through a first junction 111b between the clutch rotor 102 and the armature 103. The magnetic flux M in the armature 103 gets into the flange portion 102c through a second junction 111c between the armature 103 and the flange portion 102c. The magnetic flux M in the flange portion 102c gets again into the armature 103 through a third junction 111d between the flange portion 102c and the armature 103. The flux M in the armature 103 gets into the inner tube portion 102b through a fourth junction 111e between the armature 103 and the inner tube portion 102b. The flux M in the inner tube portion 102b returns to the ring field 105 through a gap 111f between the inner tube portion 102b and the field 105. Thus, a closed magnetic circuit is formed in the clutch rotor 102 and the armature 103.
When this magnetic circuit is formed in this manner, the clutch rotor 102 and the armature 103 frictionally engage each other in a manner such that their respective opposite surfaces are at least partially in contact with each other. This frictional engagement causes the rotor 102 and the armature 103 to rotate integrally with each other, whereupon power is transferred between them. When the power supply to the exciting coil 104 is interrupted, the magnetic flux M is canceled, so that the armature 103 is separated from the clutch rotor 102, whereupon the power transmittal is interrupted.
When the clutch rotor 102 and the armature 103 are connected magnetically to each other, first, second, third, and fourth magnetic pole portions 112a, 112b, 112c and 112d are formed on the rotor 102, ranging successively from the outer peripheral side to the inner peripheral side in the radial direction, as shown in FIGS. 8 and 9. The second magnetic pole portion 112b is supported on the outer tube portion 102a by means of the outside bridge portions 108a to 108f. The third magnetic pole portion 112c is supported on the inner tube portion 102b by means of the inside bridge portions 109a to 109f. The first magnetic pole portion 112a is supported on the outer tube portion 102a throughout its circumference. The fourth magnetic pole portion 112d is supported on the inner tube portion 102b throughout its circumference. Thus, the second and third magnetic pole portions 112b and 112c are lower in stiffness than the first and fourth magnetic pole portions 112a and 112d. Besides, the second and third magnetic pole portions 112b and 112c are formed on the flange portion 102c that is thinner than the tube portions 102a and 102b. The thin-walled flange portion 102c tends to be deformed into an outwardly convex undulatory shape by springback or the like that is caused when it is worked.
The pressure of contact between the flange portion 102c and the armature 103 is liable to increase when the clutch rotor 102 and the armature 103 are connected magnetically to each other. As mentioned before, moreover, the second and third magnetic pole portions 112b and 112c have low stiffness and easily become undulatory. When the rotor 102 and the armature 103 of the electromagnetic clutch 101 are connected magnetically to each other, therefore, the rotor 102 easily undergoes self-excited vibration. Thus, the second and third magnetic pole portions 112b and 112c vibrate during clutch operation, and noise is produced depending on the level of the vibration.
Accordingly, the object of the present invention is to provide a low-noise electromagnetic clutch subject to less vibration during clutch operation.
In order to achieve the above object, an electromagnetic clutch according to the present invention comprises a clutch rotor having a flange portion, an armature opposed to the flange portion, and an exciting coil held in the clutch rotor and adapted to form in the clutch rotor and the armature a magnetic flux for attracting the armature to the flange portion when supplied with current. The flange portion includes arcuate outside slits and arcuate inside slits, formed individually on the respective circumferences of a plurality of concentric circles, and a plurality of ring magnetic pole portions formed separated in the radial direction of the flange portion by the outside slits and the inside slits. The flange portion has a step portion and a concave surface continuous with the step portion, formed in a region corresponding to a specific one of the magnetic pole portions formed between the outside slits and the inside slits so as to be situated remoter from the armature than the other magnetic pole portions, the step portion serving to restrain the specific magnetic pole portion from touching the armature when the magnetic flux is formed.
According to the present invention, the specific magnetic pole portion of the flange portion is restrained from touching the armature in a state such that the armature is attracted to the flange portion by means of the magnetic flux generated by the exciting coil. Therefore, the magnetic pole portion of the flange portion, which has low stiffness and is liable to have an outwardly convex undulatory shape, and the armature can avoid touching each other during clutch operation. Thus, vibration of the flange portion during the clutch operation and production of noise attributable to such vibration can be restrained. In carrying out the present invention, it is to be desired that the concave surface should be formed in a region corresponding to the magnetic pole portion on the outer peripheral side, among other magnetic pole portions formed between the outside and inside slits. According to this arrangement, the region corresponding to the magnetic pole portion of the flange portion on the outer peripheral side, which is liable to vibration, never touches the armature, so that the vibration of the flange portion and the production of noise during the clutch operation can be restrained more effectively.
In order to achieve the above object, moreover, the electromagnetic clutch of the invention may be designed so that a smooth surface with surface roughness and waviness of 10 xcexcm or less is formed in a region corresponding to the specific magnetic pole portion between the outside and inside slits, in place of the step portion and the concave surface of the flange portion. According to this invention, the specific magnetic pole portion of the flange portion touches the smooth surface in a state such that the armature is attracted to the flange portion by means of the magnetic flux generated by the exciting coil. If the magnetic pole portion of the flange portion, which has low stiffness and is liable to have an outwardly convex undulatory shape, touches the armature, therefore, exciting force that is transferred from the armature to the flange portion can be reduced, so that the vibration of the flange portion and the production of noise can be restrained. In carrying out the present invention, it is to be desired that the smooth surface should be formed in a region corresponding to the magnetic pole portion on the outer peripheral side, among other magnetic pole portions formed between the outside and inside slits. According to this arrangement, the region corresponding to the magnetic pole portion of the flange portion on the outer peripheral side, which is liable to vibration, is formed of smooth surface, so that the vibration of the flange portion and the production of noise during the clutch operation can be restrained more effectively.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.